Modern communications systems continue to place ever-increasing performance demands on communications devices. Handheld communications devices in particular are subject to increasingly rigorous demands of smaller size and increased efficiency. At the same time, consumers expect these devices to have a continuously growing set of features.
For example, many handheld phones today are so-called "dual-use" phones. Dual-use phones allow users to communicate with two distinct types of communications systems. Most often, the different communications systems require different modulation formats, and these formats have widely varying dynamic ranges.
Along with features such as dual-use, consumers expect phones to be smaller, lighter, and to have longer talk times. Unfortunately, these desirable features often represent competing demands to be satisfied by the phone designer. For example, the simplest method of increasing talk time is to increase battery size, but this works against the goal of smaller size. One method of achieving increased talk time without increasing overall size is to make the device more efficient. This way, talk time as well as other desirable features can be enhanced without increasing the battery size.
Because the power amplifier is by far the largest consumer of power in handheld communications devices, increasing the efficiency of the power amplifier is very desirable. Increased power amplifier efficiency results in the ability to make smaller phones that have more features, including increased talk time. The demand for higher performance communications devices, and in particular, smaller dual-use phones with increased talk time, presents the phone designer with a difficult problem: how to design a power amplifier capable of operating efficiently for a number of different modulation formats and over a wide dynamic range.
One known method of increasing power amplifier efficiency is to use a Doherty-type power amplifier. Doherty-type amplifiers achieve an efficiency advantage over standard class AB and class B amplifiers near peak power, in part, by combining the output of two amplifiers which are biased differently. At low power levels, a first amplifier (carrier amplifier) is biased to operate linearly and a second amplifier (peaking amplifier) is off. At medium power levels, the carrier amplifier saturates while the peaking amplifier begins to turn on.
In this medium power region, the efficiency of the carrier amplifier remains at its maximum value since the carrier amplifier is saturated. The efficiency of the peaking amplifier increases from half of its maximum value at the transition point to its maximum value at system peak envelope power (PEP). As a result, the Doherty system achieves maximum efficiency at both the transition point and system PEP, and it remains relatively high in between.
The efficiency improvement of the Doherty-type amplifier is achieved from a medium power level on up, mainly because the carrier amplifier is saturated and is therefore operating very efficiently. Although considerably more efficient than a single, linear, non-saturating amplifier when operating at medium to high power levels, Doherty-type amplifiers are still quite inefficient at low power levels. When the peaking amplifier is off and the carrier amplifier is being driven by low level signals, the carrier amplifier operates in a linear region, and it is thus still inefficient.
Accordingly, there is a significant need for an efficient power amplifier capable of maintaining high efficiency while operating linearly over a wide dynamic range. There is also a significant need for a power amplifier capable of operating efficiently for a variety of modulation formats.